In many gaseous carriers, such as air, exhaust gases or the like, there are particles having a smaller size than can be directly and immediately detected by conventional detection means, such as particle counters or aerosol spectrometers. However, the detection of such particles is important, because they can have a considerable influence, e.g. in the respiratory air on the health, effectiveness and service life of filters. To enable such particles to be detected, they are “enlarged” by condensing on steam and they then function as condensation nuclei.
There is normally a storage vessel with a substance to be evaporated. The substance is brought into a humidifier means with the aid of a porous material such as felt and the like and in the same the substance evaporates. The aerosol to be measured is then passed with the steam into a condensing tube and the steam condenses on the aerosol particles and enlarges the latter. It is possible to work with external cooling and optionally also with aerosol injection or with a turbulent mixture. The apparatus and therefore also the method are sensitive for “enlargement”.
In addition, optical aerosol spectrometers are calibrated, preferably by monodisperse droplets in the range 0.2 to 8 μm. Use is made of a Sinclair-LaMer generator for producing such droplets or particles. Initially very small particles with a diameter smaller than 100 nanometers are generated as condensation nuclei and are passed through a heated, saturated steam atmosphere. Said steam-particle mixture is then passed through a cooled condensing tube in which the steam substance condenses on the nucleus. These method apparatuses are very complicated and also fault-prone, particularly if specific, precise particle sizes are required.
The problem of the invention is to provide methods and apparatus for humidifying gas, particularly air, and for condensing steam on condensation nuclei, which are simple, reliable and insensitive.